Over the past several decades, much investigation has been focused on understanding the mechanisms of friction, particularly as arising with respect to two contacting sliding surfaces. The causes of friction between sliding surfaces is discussed, for example, in the article “The Genesis of Friction” by N. P. Suh and H. C. Sin, WEAR, 69 (1981) 91-114. In general, the friction coefficient is not an inherent material property, but is composed of three principal components: one due to the deforming asperities (roughness) of the surface, another due to plowing of the surface by wear particles, and another due to adhesion. Further study has shown that the significance of wear effects can be severe for many materials, such as plastic materials, wherein interfacial wear debris is generated between the two contacting surfaces. The presence of interfacial debris has been shown to have significant adverse effects on the friction coefficient as well as on other related wear behavior. For example, the interfacial wear debris derived from polymer-based materials can contribute appreciably to the total friction coefficient.
Attempts to reduce the coefficient of friction and maintain a constant coefficient of friction between two substrates have been reported. One approach involves applying a lubricant, or grease-like substance between the sliding surfaces. Although such approaches work well in some instances, in many applications the presence of such lubricants, either separately or incorporated in the material itself, cannot be tolerated since they may introduce undesired contaminants or other undesired physical characteristics into the process or device in which the sliding surfaces are used. For example, the use of lubricants in food processing, photocopying, or orthopedic applications is not constructive owing to contamination concerns. Hence, it is desirable that a technique be developed for avoiding the use of lubricants while still maintaining a substantially constant coefficient of friction over a reasonable length of time, particularly where deleterious amounts of wear debris are generated during use.
The need for external lubricants may be reduced or eliminated by the use of polymeric contacting components. Polymeric contact components may be fabricated by applying a film of polymeric material to the surface of a substrate. Alternatively, polymeric components may be manufactured by injection molding to form intricately shaped components such as gears, cams, bearings, slides, ratchets, pumps, electrical contacts and prostheses. Polymeric contacting components provide an economical and essentially maintenance free method to reduce friction between two sliding contacting surfaces. Components formed from polymeric compounds have greater shock and vibration dampening, reduced weight, enhanced corrosion protection, decreased running noise, decreased maintenance and power use, and allow increased freedom of component design over non-polymeric components.
U.S. Pat. No. 4,174,358 discloses toughened thermoplastic compositions having a polyamide matrix resin and at least one branched or straight chain toughening polymer. The polymer may be elastomeric or thermoplastic. Examples of suitable toughening polymers include synthetic and natural rubbers such as styrene/butadiene rubber, isobutylene, isoprene, natural rubber, ethyl acrylate, butyl acrylate rubbers, etc.
U.S. Pat. No. 4,371,445 discloses tribological systems of plastic/plastic pairings in which at least one of the partners is a plastic containing polar, cyclic compounds. The cyclic part of the molecule on at least one side may be coupled directly to an atom of Group V or Group VI of the Periodic Table or the ring may contain atoms of Group V or VI. An optional auxiliary sliding partner may be formed from a polyalkylene. A partner may consist of several materials, such as a mixture of two or more of polyethylene, polypropylene, polyisobutylene, polystyrene, polytetrafluoroethylene and polyvinylidene chloride.
U.S. Pat. No. 4,987,170 discloses a styrene resin composition including styrene polymer, dimethylsilicone oil and a maleic anhydride monomer or a maleic anhydride-styrene copolymer. The styrene polymer may be modified with a rubber-like polymer, such as polybutadiene, styrene-butadiene copolymer, butadiene-acrylonitrile copolymer, ethylenepropylene-diene terpolymers and butadiene-acrylate copolymers.
One class of polymers that could be used to improve sliding properties and increase wear resistance between two sliding contacting surfaces is polyelectrolyte multilayers (PEMs). These polymers may be prepared using a layer-by-layer assembly technique introduced by Decher. See Decher, G.; Hong, J.-D. Makromol. Chem., Macromol. Symp. 1991, 46, 321-327; Decher, G.; Hong, J.-D. Ber. Bunsenges. Phys. Chem. 1991, 95, 1430-1434; and Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/211, 831. This approach, which utilizes electrostatic interactions between oppositely charged polyion species to create alternating layers of sequentially adsorbed polyions, provides a simple and elegant means of depositing layer-by-layer sub-nanometer-thick polymer films onto a surface using aqueous solutions. Lvov, Y. M.; Decher, G. Crystallography Reports 1994, 39, 628-647; Ferreira, M.; Rubner, M. F. Macromol. 1995, 28, 7107-7114; and Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171-2176. Recently, PEMs have been used in electroluminescent LEDs, conducting polymer composites, assembly of proteins and metal nanoparticle systems, thin film optoelectronic devices, and nanostructured thin film coatings. See Decher, G. Science 1997, 277, 1232; Tian, J.; Wu, C. C.; Thompson, M. E.; Sturm, J. C.; Register, R. A.; Marsella, M. J.; Swager, T. M. Adv. Mater. 1995, 7, 395; Baur, J. W.; Kim, S.; Balanda, P. B.; Reynolds, J. R.; Rubner, M. F. Advanced Materials 1998, 10, 1452-1455; Ferreira, M.; Cheung, J. H.; Rubner, M. F. Thin Solid Films 1994, 244, 806-809; and Ariga, K.; Lvov, Y.; Onda, M.; Ichinose, I.; Kunitake, T. Chemistry Letters 1997, 125-126. Despite the significant interest in polyelectrolyte multilayer films, the tribological properties of these materials have not been thoroughly investigated. Consequently, the present invention reveals that polyelectrolyte multilayer films display a significant capacity for wear prevention, thereby helping to solve the longstanding problem of reducing wear associated with movement along two surfaces.
Surface engineering, via thin organic films offers the potential for wear reduction in total joint replacement prostheses. Orthopedic implants, with metal-on-plastic configurations, have employed an ultra-high molecular weight polyethylene (UHMWPE) bearing surface for almost four decades. The life of the implant is limited by the wear of UHMWPE; the wear debris induces bone resorption through a biological reaction, causing implant loosening, and necessitating a revision surgery. Currently, no satisfactory procedures have been determined to combat the wear problems of orthopedic implants comprising UHMWPE. Therefore, it would be highly desirable to develop a highly durable polymeric film that could be used to coat joint replacement prostheses.
Surface modification has also received a large amount of attention in the case of micromotors, gear trains, mechanical relays, valves, and other devices in MEMS. See S. Sundararajan and B. Bhushan, Micro/nanoscale tribology of MEMS materials, lubricants and devices, in: B. Bhushan (Ed.), NATO advances study institute on fundamentals of tribology and bridging the gap between the macro- and micro/nano scales, Kluwer Academic Publishers, Keszthely, Hungary, 821-850, 2000. The factors impeding reliable operation of these devices include wear of the silicon-based materials, high friction forces, and stiction between the mating surfaces. See K. Komvopoulos Wear 1996, 200, 305-327. A number of organic coatings have been studied with respect to friction, stiction, and wear reduction in these devices, including Langmuir monolayers (See V. V. Tsukruk, V. N. Bliznyuk, J. Hazel, D. Visser and M. P. Everson Langmuir 1996, 12, 4840-4849), self-assembled monolayers (See S. Sundararajan and B. Bhushan, Micro/nanoscale tribology of MEMS materials, lubricants and devices, in: B. Bhushan (Ed.), NATO advances study institute on fundamentals of tribology and bridging the gap between the macro- and micro/nano scales, Kluwer Academic Publishers, Keszthely, Hungary, 821-850, 2000; and V. DePalma and N. Tillman Langmuir 1989, 5, 868-872), and polymer films with layered architectures. See D. Julthingpiput, H. Ahn, D. Kim and V. V. Tsukruk Tribology Letters 2002, 13, 35-40 and A. Sidorenko, H. Ahn, D. Kim, H. Yang and V. V. Tsukruk Wear 2002, 252, 946-955. Most of these coatings require either an intricate protocol for assembly, or a large density of functional groups by which they can covalently bind to the substrate. Release of corrosive by-products, and polymerization of precursor molecules leading to particulate formation, are some examples of other problems encountered during processing of these films. See R. Maboudian, W. R. Ashurst and C. Carraro Tribology Letters 2002, 12, 95-100 These issues are not encountered during processing of PEMs. Consequently, PEMs may present a facile solution to the tribological issues facing MEMS.